PostgreSQL continuous archiving not running archive_command

I've been trying to set up continuous archiving for a simple, test PostgreSQL 9.0 database, as per the documentation. In postgres.conf I've set:

wal_level = archive
archive_mode = on
archive_command = 'touch /home/myusername/backup/testtouch'
archive_timeout = 30s

…and restarted PostgreSQL. The file listed by touch never appears. I can manually run the touch command and it works as expected.
If I try to create a backup, it waits forever for the archive_command. In psql;

postgres=# SELECT pg_start_backup('touchtest');
pg_start_backup
—————–
0/14000020 (1 row)

postgres=# SELECT pg_stop_backup();
NOTICE: pg_stop_backup cleanup done, waiting for required WAL segments to be archived > WARNING: pg_stop_backup still waiting for all required WAL segments to be archived (60 seconds elapsed)
HINT: Check that your archive_command is executing properly. pg_stop_backup can be cancelled safely, but the database backup will not be usable without all the WAL segments.

What would cause this? How can I troubleshoot it?

Additional info: Running on CentOS 5.4. PostgreSQL 9.0.2 installed as root.

Update: I first tried archiving with both cp -i %p /home/myusername/backup/%f </dev/null and test ! -f /home/myusername/backup/%f && cp %p /home/myusername/backup/%f to match the manual. I reduced it to the simpler touch call for troubleshooting.